It is known that an analysis of ionizable compounds, such as proteins, can be made by subjecting a sample, of for example blood, to an electrical potential as taught in U.S. Pat. Nos. 3,407,133 (Oliva et al.), 3,479,265 (Elevitch) and 3,875,045 (Bergrahm et al.). The sample to be analyzed by electrophoresis is placed on a suitable support medium, such as a gel, of the types disclosed in U.S. Pat. No. 3,725,004 (Johnson et al.). Such support medium may include, for example: (1) aqueous solutions of agar or agarose as disclosed in U.S. Pat. Nos. 3,281,409 (Blethen), 3,335,127 (Polson), 3,362,884 (Morse) and 3,766,047 (Elevitch); (2) synthetic polymeric gelling agents as disclosed in U.S. Pat. No. 3,046,201 (White et al.); and (3) cellulose and cellulose acetate as disclosed in U.S. Pat. No. 3,360,440 (Haab et al.). However, such containers have required secure retention of the electrophoretic support medium within the container to prevent the support medium from coming in contact with any part of the container to prevent flaws, such as marks or cracks, from being formed in the support medium. Such flaws are aggravated by shrinkage of the support medium caused by dehydration as disclosed, for example, in U.S. Pat. No. 4,314,897 (Monte et al.).